I was just reading "Fred Wilson: Facebook's Worth $5- $7 Billion" over on Silicon Alley Insider and thought I'd come up with something witty to say in response, but I imagine that Fred Wilson knows more than I.
However, what I do know is that Zuckerberg is probably thinking in his own mind that Facebook is now worth perhaps $16b, $17b, ..., $20b. He's like a homeowner who thinks their $175k house is worth $275k.
People like me think that Facebook is worth $1b, if that. The kids who use Facebook aren't there to click on ads. Rather, it's basically a gossip site (and a few other things like posting pics of drunk people at parties or about guys who skip work and dress up a fairies at a party and later get fired for this). In any case, they're not there to consume or click on ads. They are college kids and (perhaps) they get their college paid for. Thus, Facebook has a low CPM and at best might be worth $1b.
The only reason one could argue they are valued more is ad deals that really don't make much sense. I agree with Wilson that Microsoft overpaid for their micro-investment in Facebook though.